1. Field of the Invention
The present invention relates generally to an optical device having a function of an optical circulator and, more particularly, to a three-port optical circulator of a novel structure, an optical add/drop circuit having the three-port circulator, and a four-port optical circulator of a novel structure.
2. Description of the Related Art
An exemplary four-port optical circulator known heretofore is disclosed in, for example, "A Compact Polarization-Independent Optical Circulator", THE TRANSACTIONS OF THE IECE OF JAPAN, Vol. E64, No. 1, January 1981, pp. 30-31. First a brief description will be given below on the structure and operation of the above four-port optical circulator with reference to FIG. 1. This optical circulator has four ports 1 to 4. And a triangular prism 6, a quadrangular prism 8, a quadrangular prism 12 and a triangular prism 10 are disposed opposite to the four ports 1 to 4, respectively. The prisms 6, 8, 10 and 12 are each composed of double refractive crystal such as rutile. The prisms 6 and 10 are mutually the same in shape, and also the prisms 8 and 12 are mutually the same in shape. The prisms 6 and 12 are fixed to each other via a thin air layer, and similarly the prisms 8 and 10 are fixed to each other via a thin air layer. A Faraday rotor 14, which is composed of magneto-optical crystal such as YIG and to which a magnetic field H is applied from an unshown magnet, and a half-wave plate 16, are disposed in this order between one set of the prisms 6 and 12 and another set of the prisms 8 and 10. The Faraday rotor 14 gives a 45.degree. polarization plane rotation to an input beam clockwise with respect to the magnetic field H. The half-wave plate 16 gives a 45.degree. polarization plane rotation to an input beam counterclockwise with respect to a direction of propagation of the input beam. Consequently, the set of the Faraday rotor 14 and the half-wave plate 16 gives a 90.degree. polarization plane rotation to the beam which propagates leftward from the right in FIG. 1 while giving none of polarization plane rotation to the beam propagating rightward from the left in FIG. 1.
In this specification, an optical path from the port 1 toward the port 2 is termed a path (1, 2); an optical path from the port 2 toward the port 3 is termed a path (2, 3); an optical path from the port 3 toward the port 4 is termed a path (3, 4); and an optical path from the port 4 toward the port 1 is termed a path (4, 1). Contrary to the above, an optical path from the port 2 toward the port 1 is termed a path (2, 1); an optical path from the port 3 toward the port 2 is termed a path (3, 2); an optical path from the port 4 toward the port 3 is termed a path (4, 3); and an optical path from the port 1 toward the port 4 is termed a path (1, 4).
The paths (1, 2), (2, 3), (3, 4) and (4, 1) are in the forward direction, and it is demanded in each of the forward paths that the loss be minimum and not dependent on the state of polarization. Meanwhile the paths (2, 1), (3, 2), (4, 3) and (1, 4) are in the reverse direction, and it is demanded in each of the reverse paths that the loss is sufficiently great.
In FIG. 1, the path (1, 2) and the path (2, 3) will be explained below.
The beam supplied from the port 1 is separated, at a junction plane of the prisms 6 and 12, into an S polarized beam of reflected component and a P polarized beam of transmitted component. The S polarized beam is supplied to the prism 8 by way of the prism 6, the Faraday rotor 14 and the half-wave plate 16 in this order. Since the polarization state of this beam is not converted, the beam is reflected by the junction plane of the prisms 8 and 10 and then is outputted to the port 2. Meanwhile the P polarized beam is supplied to the prism 10 by way of the prism 12, the Faraday rotor 14 and the half-wave plate 16 in this order. Since the polarization state of this beam is not converted either, this beam is transmitted through the junction plane of the prisms 10 and 8 and then is outputted to the port 2.
The beam inputted to the port 2 is separated, at the junction plane of the prisms 8 and 10, into an S polarized beam and a P polarized beam. The S polarized beam is supplied to the prism 6 by way of the prism 8, the half-wave plate 16 and the Faraday rotor 14 in this order. Since this beam is given a 90.degree. polarization plane rotation, it is transmitted through the junction plane of the prisms 6 and 12 and then is outputted to the port 3.
The P polarized beam is supplied to the prism 12 by way of the prism 10, the half-wave plate 16 and the Faraday rotor 14 in this order. Since this beam also is given a 90.degree. polarization plane rotation, it is reflected by the junction plane of the prisms 12 and 6 and then is outputted to the port 3.
As it will be understood that a similar operation is performed with regard to any of the other paths, an explanation thereof is omitted here.
Recently, there is proposed an optical add/drop circuit having of a combination of an optical circulator and a fiber grating. A fiber grating reflects light of a specific narrow band while transmitting light of any other wavelength outside of such a band. As will be described later, an optical add/drop circuit for practical use is to be equipped with a three-port optical circulator. For example, in providing a three-port optical circulator by the use of a four-port optical circulator as shown in FIG. 1, a process of total reflection may be executed with respect to the port 4. In this case, however, the forward optical path from the port 3 toward the port 1 includes the paths (3, 4) and (4, 1), so that theoretically the forward loss becomes more than double. Further, since the reverse optical path from the port 1 toward the port 3 includes the total reflection plane (port 4), the reflected component based on the input beam to the port 3 is outputted therefrom to consequently deteriorate the characteristic.
An exemplary three-port optical circulator is disclosed in Japanese Patent Laid-open No. Hei 4-216522. In this optical circulator, there exists a problem that some difficulties are unavoidable in manufacture due to the necessity of mutually reversing the rotational directions of polarization planes of two Faraday rotors which are disposed at relatively proximate positions.